The class of diaryl ethers finds diverse industrial use such as multifunctional gas additives, antioxidants, heat transfer fluids, and flame retardants, to name but a few. Unfortunately, with some exceptions pertinent to unusually reactive systems, there are few commercially feasible general preparatory methods. Perhaps the most common example of diaryl ether synthesis using activated reactants is the base-catalyzed condensation of phenols with ortho- or para-nitrosubstituted halobenzenes, whose halogens are unusually susceptible to displacement, to afford the corresponding nitro-substituted diaryl ethers.
The more common methods of preparing diaryl ethers require rather severe conditions. For example, the reaction of phenols with unactivated aromatic halides, such as bromobenzene, occurs via a benzyne intermediate and this requires the use of a rather strong base, which presents increasingly unacceptable environmental problems associated with base disposal. Copper salts have been reported to catalyze this reaction (Chem. Abst., 79, 104842 (1973)) but the propensity of copper salts to catalyze oxidative coupling of phenol (see, for example, U.S. Pat. No. 4,070,383) will be a complicating factor. Japanese Kokai Tokkyo Koho JP 63072640 relates to the preparation of a diphenyl ether by treating phenol or alkylated phenols with certain crystalline metal-substituted silicates, such as those of the ZSM-type. However, the self-condensation of phenol to give diphenyl ether occurs at 200.degree. C. with only 2% conversion and 96% selectivity after 20 hours reaction time. U.S. Pat. No. 4,360,699 describes oxygen-carbon coupling between the phenolic hydroxyl and the carbon of an aromatic ring as catalyzed by aluminum at 300.degree.-375.degree. C. Somewhat more pertinent to the work described within is Japanese Kokai Tokkyo Koho JP 59196835 which discloses reacting a mixture of phenol, water, and benzene in the ratio of 1:0.2:3 in the gas phase over powdered titania or zirconia at 400.degree. C. to give diphenyl ether with 67% selectivity and 42% conversion.
For various reasons the industrially preferred preparation of diphenyl ether has been the dehydration of phenol as catalyzed by thoria. The radioactivity of thoria provides an impetus for its replacement as a catalyst in an industrial process.
A good etherification catalyst should be easily compounded from readily available materials, should exhibit a good lifetime, and should not be an environmental hazard nor present environmental problems upon its disposal. We have found that if a composite of tungsten oxide supported on certain other inorganic oxides is partially reduced with hydrogen, the resulting material serves as an effective catalyst in the dehydration of phenols to give diaryl ethers. This method of preparing diaryl ethers is applicable to a diversity of phenols.